1. Field of the Invention
The invention relates to overload detection circuits for the protection of electrical motors and other electrical equipment.
2. Description of the Prior Art
One prior overcurrent detection device is disclosed in Grzebielski, U.S. Pat. No. 4,327,391. That device senses current that exceeds 100% of rated current for a DC motor. The device uses an integrator circuit to generate a sawtooth signal that is converted to a pulse train by repeatedly discharging a capacitor through a monostable multivibrator circuit. The pulses are coupled to a pair of cascaded counters which count up in response to an overcurrent condition and generate a fault signal. The fault signal is coupled to a latch for operation of a relay that disconnects power to the motor.
In another prior overcurrent detection device the sawtooth output signal from the integrator circuit is used with one up-counting comparator and one down-counting comparator to generate the pulse train. This type of pulse generator eliminates the need for some of the components used in Grzebielski, such as a capacitor that is discharged through the multivibrator circuit. In using the integrator circuit to more directly generate pulses there is a technical problem in resetting the integrator circuit without creating a time delay between pulses, and it has not been apparent how to accomplish this.
In both of the prior circuits referred to above, there has been but a single threshold for generating overcurrent pulses, and this threshold has typically been set at or near 100% of rated current for the protected electrical device. In today's products, there is an increasing need to provide a more sophisticated response to different overcurrent conditions, and it is believed that the present invention contributes significantly in satisfying that requirement.